fix(plex,channels): address R13 S2 review findings

- Shared usePlexClearFilters hook replaces the duplicated clearAll (was copied in
  PlexBrowse and PlexSidebar) — one "reset every Plex narrowing" both call.
- The Plex duration chip's minutes unit is localized (plex.filter.durationUnit:
  "min"/"perc") instead of a hardcoded "m".
- The Plex collection chip falls back to the "Collection" label, not the raw
  rating_key id, when collectionTitle is missing.
- Dropped the dead rowClassName that dimmed hidden rows in the subscribed table
  (hidden channels are excluded there now — they live in the Hidden tab).
- Renamed the misleading `set` null-check predicate to `has`.

Gate green: tsc/eslint-0/prettier/knip clean, vitest 106.

frontend/src/components/Channels.tsx

@@ -578,7 +578,6 @@ export default function Channels() {
               controlsBandClassName="px-4 max-w-384 mx-auto"
               sort={sort}
               onSortChange={setSort}
-              rowClassName={(c) => (c.hidden ? "opacity-60" : "")}
               emptyText={t("channels.empty")}
             />
           ) : (
@@ -589,7 +588,6 @@ export default function Channels() {
               leadKey="priority"
               aboutKey="about"
               actionsKey="actions"
-              rowClassName={(c) => (c.hidden ? "opacity-60" : "")}
               emptyText={t("channels.empty")}
             />
           )}